1. Field of the Invention
The present invention relates to novel compositions especially adapted for enhancing the properties of vulcanized substrates, to a process for the preparation thereof, and to the use of said novel compositions particularly for the strengthening/reinforcing of natural and synthetic rubbers.
2. Description of the Prior Art
Research has long been carried out in this art in quest of additives to improve the properties of elastomers reinforced with mineral fillers, and particularly the properties of silica/rubber compositions.
It has been observed that the use of compounds containing amino groups in silica/rubber compositions vulcanized with sulfur improves the preparation and properties of the vulcanized materials; see, for example U.S. Pat. Nos. 2,692,870, 2,692,871 and 3,399,166, and French Pat. No. 1,436,625.
In U.S. Pat. No. 3,499,865 a preliminary reaction between the sulfur and the organic amines takes place in the presence of a portion of the filler used in the rubber (preparation of a masterbatch).
In U.S. Pat. No. 3,994,742 compounds containing both amino groups and sulfur groups are used. These are defined as being coupling agents, for the same reason as the well known derivatives of the mercapto-silane family, one representative of which, .gamma.-mercaptopropyl trimethoxysilane, is acknowledged to be one of the most effective in this field (French Pat. No. 2,094,859).
U.S. Pat. Nos. 4,151,157 and 4,156,677 describe compounds which combine the three amino, sulfur and trialkoxysilane groups in the same structure.
Thus, it will be seen that very considerable research has been carried out in this field, but that the solutions proposed are not entirely satisfactory, either for economic reasons because the additives are still too expensive relative to the filler proper, as in the case of the mercapto-silanes, or for practical reasons because the additives are not effective enough or are difficult to prepare.
The problem is indeed extremely complex. On the one hand the task of mixing for vulcanization has to be facilitated, a step commonly described as aid processing, and on the other hand the properties of the vulcanized material have to be improved. In the vulcanizing process one, therefore, has to take into account not only the compatibility and particularly the solubility of the compounds added to the elastomeric matrix but also its behavior vis-a-vis the filler. This behavior must be known both from the chemical point of view and from the point of view of the structure or morphology of the filler.
It is also necessary to provide a composition which has sufficient stability at temperatures below vulcanization temperature, and which are active under vulcanizing conditions and in the vulcanization medium.
This explains why some compositions bearing both sulfur and nitrogen functions are not effective, and why it was found preferable to introduce the nitrogen function and the sulfur function separately.
It also explains why, despite considerable work done on the subject for several decades and despite the large number of additives proposed, particularly in the silane family, better solutions to the problem are still being researched.
Applicants have thus been led to take a different approach, breaking away from the traditional one, and using what they deem "interface agents".
This term is used in the sense of an additive containing:
(1) at least part of a molecule which has more affinity to silica than to the other ingredients of the rubber mix; PA1 (2) an alkyl group compatible with the elastomer. PA1 (i) at least one mercaptan group, --SH PA1 (ii) at least one polysulfide group, --S.sub.x --, with x.gtoreq.2 PA1 (1) reacting the mercaptoester of the formula HS(CH.sub.2).sub.n CO.sub.2 CH.sub.3, preferably with n=1 to 10, with the TEPA, advantageously at about 30.degree.-90.degree. C., with or without a diluent, the diluent preferably being an aromatic hydrocarbon such as xylene or toluene. PA1 (2) Eliminating the alcohol formed by conventional methods, while taking care to maintain the above temperatures, although the alcohol may equally be left in the medium in order to simplify the process. PA1 (a) preparing a mercaptoacid in known manner (adding thioacetic acid to a mono-unsaturated acid, then saponifying it in accordance with J. Org. Chem., 23, 1525 (1958)); PA1 (b) esterifying the mercaptoacid with an alcohol such as methanol. In this manner, one produces an aliphatic ester of the formula: EQU HS(CH.sub.2).sub.n CO.sub.2 CH.sub.3 n=3 to 10 PA1 (1) reacting sulfur with a mono-unsaturated ester to obtain a sulfurized ester, the S/ester ratio being a minimum 2 and a maximum 6 depending upon the quantity of sulfur used (2 to 6 g/atoms of sulfur per mole of ester) and the reaction conditions (temperature 140.degree. C.-160.degree. C., duration 3 hours to 16 hours); PA1 (2) reacting the sulfurized ester with the TEPA by heating it to 30.degree.-90.degree. C. with or without a solvent in the form of xylene or toluene. PA1 (3) removing the alcohol formed in the usual manner or permitting it to remain in the reaction medium. PA1 Silane: gamma-mercaptopropyl-trimethoxysilane, marketed by Union Carbide as A-189. PA1 (i) a white solid (N,N'-dicyclohexylurea) which was washed with cold methylene chloride and dried (weight 139.6 g or 96% theory, melting point 232.degree. C.); PA1 (ii) a yellow filtrate, to which the solvent from the previous wash was added. When all of the methylene chloride had been removed in a rotating evaporator, 126 g of light brown substance were obtained, which substance was viscous when cold and which contained 16% of sulfur. Infra-red analysis confirmed the presence of amide bonds. PA1 (a) minimum torque (mT): consistency of non-vulcanized mix ("raw" mix) at testing temperature; PA1 (b) maximum torgue (MT): consistency of mix after cross-linking; PA1 (c) .DELTA. torque: MT-mT is related to cross-linking rate; PA1 (d) precocity: time required to begin cross-linking at testing temperature; PA1 (e) index: related to vulcanizing speed (optimum time-precocity); PA1 (f) optimum time: ##EQU1## PA1 (a)--ASTM D 412-51 T PA1 (b)--ASTM D 2240-75 PA1 (c)--French Standard T 47-126 PA1 (d)--DIN 53516 PA1 (e)--ASTM D 1054-55 PA1 ASTM D 623-67 PA1 (a) static compression (SC%): deflection under constant load; PA1 (b) permanent deformation (PD%): % of residual deformation after test; PA1 (c) dynamic comression (DC%): % of deformation at equilibrium during test PA1 (d) .DELTA. T. base: .DELTA. T. between temperature at surface of sample (at its base) and temperature of chamber; PA1 (e) .DELTA. T. core: .DELTA. T. between temperature at core of sample and temperature of chamber; PA1 (f) conditions for tests:
The effect of such additive is to reduce the cohesion of the silica network.
Applicants first developed additive compositions based on alkenyl succinimides, obtained by condensing a polyamine on a succinic alkenyl anhydride in which the alkenyl radical contained from 3 to 100 and preferably 3 to 80 carbon atoms (European Pat. No. 32,076).
It was noted that these products also displayed a marked synergistic effect with conventional silanes (European Pat. No. 34,970).
Of the interface agents, tetraethylene pentamine (TEPA) derivatives have been found to be particularly effective. These derivatives are also quite inexpensive, firstly because of their ease of preparation and secondly because of the accessibility of the raw materials; the commercial TEPA used is in fact a mixture of polyamines containing varying proportions of pure TEPA.
TEPA is known as a vulcanizing agent and sulfur activator. It has unfortunately been found to be highly reactive with sulfur groups, a fact which may lead to a loss of sulfur in the form of hydrogen sulfide, H.sub.2 S.